you dont have any lights in your level. so your basically working in a dark room. two things you can do in this situation: first add in some lights such as a directional light (and maybe a skysphere and atmospheric fog). once you place a light source you may also need to rebuild the lighting. or you can change the view mode to unlit by clicking where it says lit near the top left of the viewport. changing the view mode will only affect the editor though so when you play the game you will still be in the dark.